Acoustic wave devices employing a piezoelectric thin film resonator are used as filters and duplexers of wireless devices such as mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film. A region where the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region.
Rapid diffusion of mobile phones leads to the use of many frequency bands. This requires the filter and the duplexer to have sharp skirt characteristics. For example, a filter employed in a duplexer having a narrow guard band, which is a frequency band between a transmit band and a receive band, is required to have sharp characteristics of the filter.
Japanese Patent Application Publication No. 2013-123184 (Patent Document 1) discloses providing a thick film portion having a thick multilayered film in the outer periphery of the resonance region of a piezoelectric thin film resonator and making widths of the thick film portions from the edges of the resonance regions different from each other between at least two piezoelectric thin film resonators in the filter to achieve sharp skirt characteristics.
Making the widths of the thick film portions of the resonators different from each other enables to make effective electromechanical coupling coefficients different from each other. This enables to achieve sharp skirt characteristics. However, in the structure disclosed in Patent Document 1, the capacitor of the thick film portion does not have a high Q-value, and thus skirt characteristics are not sufficiently sharp.